1. Field of the Invention
The present invention relates to foam particles of an uncrosslinked linear ethylenic resin, which have heretofore been deemed to be practically not realizable, and foam moldings prepared therefrom and methods for their preparation. More particularly, the present invention pertains to foam particles of a linear ethylenic resin which can be expansion molded by heating in an uncrosslinked state and from moldings prepared therefrom, said resin being a specific linear ethylenic resin capable of providing in-mold foam moldings which are novel and useful and possess excellent properties when compared with in-mold foam moldings of crosslinked polyethylene resins prepared by the process of the prior art.
2. Description of the Prior Art
As is well known in the art, foam moldings with complicated shapes corresponding to the shape of certain mold cavities can be produced according to a prior art process which comprises filling the mold cavity with foam particles of a synthetic resin, permitting these particles to expand thereby filling the gaps formed between adjacent particles, and simultaneously effecting close fusion between the mutually expanded particles. This process has been developed for the expansion molding of a polystyrene resin, and has been effective in the production of in-mold foam moldings of polystyrene.
However, when such a process is applied to uncrosslinked polyethylene as a substitute for the polystyrene base resin, satisfactory foam molding could not be obtained. The problem may be inherent in polyethylene resins, namely poor retentivity by the resin film of the gases generated from the blowing agent on melting (namely gas barrier characteristic) on one hand, and marked variation in the visco-elastic changes of said resin as a function of the temperature, on the other. Thus it has been very difficult to determine adequate expansion conditions for the resin.
Accordingly, in processes for producing in-mold expanded moldings of polyethylene resins, it has been proposed to use crosslinked polyethylene as the base resin through the use of a chemical crosslinking agent or radiation in order to modify the properties inherent in uncrosslinked polyethylene resin, and developments in this direction have progressed rapidly in recent years. For example, developments have been made in the processes for the preparation of foam particles or in-mold foam moldings of polyethylene resins capable of producing molding having surface smoothness and luster comparable to the in-mold foam moldings of polystyrene (see Japanese Laid-open Patent Publications Nos. 14632/1982 and 70621/1982).
These techniques concerning the in-mold expansion molding of polyethylene resins have been developed on the basis of expanding the resin used as the base material in a crosslinked state. These crosslinking techniques have been known and frequently used, primarily as means for modifying the properties of the resin mainly because they have the advantage of not impairing the properties inherent in ethylene resin as compared with means for modifying the properties of said resin in which other resin components such as polystyrene or ethylene vinyl acetate are permitted to co-exist through copolymerization or mixing.
However, the step of crosslinking polyethylene is an additional step which increases the overall cost of the process, such as installation cost or energy cost. Another disadvantage in using a crosslinked foam molding is that it cannot be regenerated and returned to its original state for reuse. Because of the demands to conserve energy and resources, it is desired to develop a technique for the in-mold expansion molding of polyethylene which omits the crosslinking step. Furthermore, the in-mold expansion molding of crosslinked polyethylene of the prior art has not been satisfactory with respect to retaining the heat resistance and adiabatic performance of the resin, or the dimensional stability and rigidity of the resin at a high degree of expansion.